The present invention relates to testing and calibration of electronic electricity meters. More specifically, the present invention relates to high speed multifunction testing and calibration of electronic electricity meters.
Calibration of electricity meters is typically verified by comparison of a meter under test (MUT) to a standard, or reference, meter for a time interval. The initiation of the time interval being signaled by the meter under test. The standard meter and MUT measure the same, or proportionate, electrical loads during the test interval. This has been the preferred way to test electricity meters for the last fifty years. With electromechanical single function electricity meters, the test interval corresponds to an exact number of revolutions of the meter disk for the MUT. With electronic meters, the test interval corresponds to some internal procedural step, or threshold. With either kind of meter, the test interval corresponds to a known quantum of electricity (known by the measurement of the standard meter) measured by the MUT.
Typically, prior to the start of the test interval, several seconds are allowed for loads to be established and meters to reach practically steady-state conditions; known as on-the-fly testing. Testing of individual elements, or stators, of polyphase meters has been carried out by a sequence of tests: one for each element and one for all elements combined. Additionally, the pulsating nature of a.c. quantities require extended test intervals to reduce their relative effects on the accuracy of comparisons where the MUT and the standard have different time characteristics, effective level of filtering, or smoothing. It is important to note, that a.c. power as watts integrated over time is sinusoidal and not a straight line. Many electronic meters track a.c. energy exactly, i.e., sinusoidally. However, many electronic standards used for meter calibration are filtered to give the appearance of smoothing out the sinusoidal. Comparing two such different devices can result in differences between their readings. This phenomenon was not an issue with electromechanical meters, as they inherently exhibited heavily filtered characteristics.
Referring to FIG. 1, herein, labeled prior art, the cyclic nature of a.c. power is shown. By way of example, two meters having equal accuracy: the first, a heavily filtered meter, integrates average power with respect to time, and the second, an unfiltered meter, integrates instantaneous power with respect to time. The energy curves of these meters are compared, e.g., during a time interval `a` to `b`.
The unfiltered meter records energy proportional to the cross hatched area. The filtered meter will record in proportion to the area bounded by the vertical lines `a/T` and `b/T`, the abscissa, and the average power line, the broken line designated `P`. In this example, the cross hatched area is obviously larger; for other values of `a` and `b`, it could be smaller or even the same.
Deviations that can occur in comparing meters are dependent on when in the voltage cycle, at `a/T`, the test starts and the test duration, defined by: (b-a)/T. As discussed hereinabove, the start is typically triggered by the meter under test (MUT) and usually not controlled. However, the prior art has addressed this problem by selecting a minimum comparison time which allows the limits of uncertainty to be managed.
By way of example, referring to FIG. 2, herein, labeled prior art, a plot is shown which is used to aid in selecting minimum test times with a desired accuracy. In one example, with an acceptable level of uncertainty of .+-.0.05%, the intersection on the 1.00 PF line is at approximately 5.3 seconds. Accordingly, the test time must be greater than 5.3 seconds at 1.00 PF at 60 Hz to obtain a 0.01% level of uncertainty in the comparison. Twice and four times as long are needed for similar uncertainty levels of 0.50% and 0.25 PF respectively. In general, with all the other variations unchanged, the longer the test interval the less the relative uncertainty.